Evidence suggests that oestrogens are the major mitogens involved in promoting the growth of tumours in endocrine-dependent tissues, such as the breast and endometrium. Although plasma oestrogen concentrations are similar in women with or without breast cancer, breast tumour oestrone and oestradiol levels are significantly higher than in normal breast tissue or blood. In situ synthesis of oestrogen is thought to make an important contribution to the high levels of oestrogens in tumours and therefore inhibitors, in particular specific inhibitors, of oestrogen biosynthesis are of potential value for the treatment of endocrine-dependent tumours.
Over the past two decades, there has been considerable interest in the development of inhibitors of the aromatase pathway—which converts the androgen precursor androstenedione to oestrone. However, there is now evidence that the oestrone sulphatase (E1-STS) pathway, i.e. the hydrolysis of oestrone sulphate to oestrone (E1S to E1), and aromatase (i.e. conversion of androstenedione to oestrone) account for the production of oestrogens in breast tumours.
FIGS. 1 and 2 are schematic diagrams showing some of the enzymes involved in the in situ synthesis of oestrone from oestrone sulphate, oestradiol and androstenedione.
In FIG. 2, which schematically shows the origin of oestrogenic steroids in postmenopausal women, “ER” denotes Oestrogen Receptor, “DHA-S” denotes Dehydroepiandrosterone-Sulphate, “Adiol” denotes Androstenediol, “E1-STS” denotes Oestrone Sulphatase, “DHA-STS” denotes DHA-sulphatase, “Adiol-STS” denotes Adiol Sulphatase, and “17B-HSD” denotes Oestradiol 17B-hydroxysteroid dehydrogenase.
As can be seen, the main two enzymes that are involved in the peripheral synthesis of oestrogens are the aromatase enzyme and the enzyme oestrone sulphatase.
In short, the aromatase enzyme converts androstenedione, which is secreted in large amounts by the adrenal cortex, to oestrone. Recent reports have suggested that some flavones could inhibit aromatase activity.
Much of the oestrone so formed, however, is converted to oestrone sulphate (E1 S) and there is now a considerable body of evidence showing that E1S in plasma and tissue acts as a reservoir for the formation of oestrone by the action of oestrone sulphatase.
In this regard, it is now believed that the oestrone sulphatase (E1-STS) pathway—i.e. the hydrolysis of oestrone sulphate to oestrone (E1S to E1) is a major source of oestrogen in breast tumours. This theory is supported by a modest reduction of plasma oestrogen concentration in postmenopausal women with breast cancer treated by aromatase inhibitors, such as aminoglutethimide and 4-hydroxyandrostenedione and also by the fact that plasma E1S concentration in these aromatase inhibitor-treated patients remains relatively high. The long half-life of E1S in blood (10-12 h) compared with the unconjugated oestrogens (20 min) and high levels of steroid sulphatase activity in liver and, normal and malignant breast tissues, also lend support to this theory.
Thus, oestrogen formation in malignant breast and endometrial tissues via the sulphatase pathway makes a major contribution to the high concentration of oestrogens which are present in these tumours. However, inhibition of both the aromatase and sulphatase pathways could offer considerable therapeutic benefit.
PCT/GB92/01587 teaches novel steroid sulphatase inhibitors and pharmaceutical compositions containing them for use in the treatment of oestrone dependent tumours, especially breast cancer. These steroid sulphatase inhibitors are sulphamate esters, such as N,N-dimethyl oestrone-3-sulphamate and, preferably, oestrone-3-sulphamate (otherwise known as “EMATE”). EMATE has the following structure: 
It is known that EMATE is a potent E1-STS inhibitor as it displays more than 99% inhibition of E1-STS activity in intact MCF-7 cells at 0.1 nM. EMATE also inhibits the E1-STS enzyme in a time- and concentration-dependent manner, indicating that it acts as an active site-directed inactivator. Although EMATE was originally designed for the inhibition of E1-STS, it also inhibits dehydroepiandrosterone sulphatase (DHA-STS), which is an enzyme that is believed to have a pivotal role in regulating the biosynthesis of the oestrogenic steroid androstenediol. Also, there is now evidence to suggest that androstenediol may be of even greater importance as a promoter of breast tumour growth. EMATE is also active in vivo as almost complete inhibition of rat liver E1-STS (99%) and DHA-STS (99%) activities resulted when it is administered either orally or subcutaneously. In addition, EMATE has been shown to have a memory enhancing effect in rats. Studies in mice have suggested an association between DHA-STS activity and the regulation of part of the immune response. It is thought that this may also occur in humans. The bridging O-atom of the sulphamate moiety in EMATE is important for inhibitory activity. Thus, when the 3-O-atom is replaced by other heteroatoms as in oestrone-3-N-sulphamate and oestrone-3-S-sulphamate, these analogues are weaker non-time-dependent inactivators.
In addition to oestrone, the other major steroid with oestrogenic properties which is produced by postmenopausal women is androstenediol (see FIG. 2).
Androstenediol, although an androgen, can bind to the oestrogen receptor (ER) and can stimulate the growth of ER positive breast cancer cells and the growth of carcinogen-induced mammary tumours in the rat. Importantly, in postmenopausal women 90% of the androstenediol produced originates from the androgen dehydroepiandrosterone sulphate (DHA-S) which is secreted in large amounts by the adrenal cortex. DHA-S is converted to DHA by DHA sulphatase, which may be the same as, or different from, the enzyme, oestrone sulphatase, which is responsible for the hydrolysis of E1S.
During the last 10-15 years considerable research has also been carried out to develop potent aromatase inhibitors, some of which are now marketed. However, in three recent reports of postmenopausal women with breast cancer who received aromatase inhibitor therapy, plasma E1S concentrations remained between 400-1000 pg/ml.
In summation therefore in situ synthesis of oestrogen is thought to make an important contribution to the high levels of oestrogens in tumours and therefore specific inhibitors of oestrogen biosynthesis are of potential value for the treatment of endocrine-dependent tumours.
Moreover, even though oestrogen formation in malignant breast and endometrial tissues via the sulphatase pathway makes a major contribution to the high concentration of oestrogens, there are still other enzymatic pathways that contribute to in vivo synthesis of oestrogen.
The present invention seeks to provide novel compounds suitable for the inhibition of steroid sulphatase activity and aromatase activity.